An electrochemical measuring cell of this kind is disclosed in U.S. Pat. No. 3,649,505 and includes a pH-electrode as a measuring electrode which is used to measure hydrogen ions. This potentiometric measurement of the ammonia concentration requires a long time duration for a completed measuring reaction. The long time duration is needed for the adjustment of an equilibrium. In this time duration, the NH.sub.3 to be detected and the water content of the electrolyte conjointly form NH.sub.4 OH which, in turn, dissociates into NH.sub.4.sup.+ ions and OH.sup.+ ions. The slow step determining the speed for this reaction is the adjustment of the equilibrium with the gas space.
The concentration of NH.sub.4.sup.+ ions is to be held approximately constant in order to carry out an ammonia measurement based on the pH-measurement. This concentration is to be held constant in order to obtain the most linear connection possible between the NH.sub.3 -partial pressure and the proton concentration to thereby suppress as much as possible disturbances of the pH-value by sources other than the ammonia concentration. For this purpose, an ammonia salt containing NH.sub.4.sup.+ is added to the electrolyte of the known measuring cell. In this way, an excess of NH.sub.4.sup.+ ions is present in the electrolyte so that with a dissociation of NH.sub.4 OH during the ammonia measurement, the NH.sub.4.sup.+ ions formed in this way no longer exercise any noticeable influence on the total concentration of NH.sub.4.sup.+ ions.
The added ammonia salt thereby makes possible a defined connection between the ammonia concentration to be detected and the OH.sup.- or H.sup.+ concentrations for a potentiometric ammonia measurement with the OH.sup.- or H.sup.+ concentrations being formed in the electrolyte.
The foregoing notwithstanding, the known measuring cell provides the following disadvantages which are peculiar to the potentiometric measurement.
The glass electrode required for the pH-measurement changes in the characteristic of the glass membrane in the course of its use so that drift phenomena occur.
A stable reference potential is necessary for carrying out the pH measurement and a displacement of this reference potential in the course of use likewise leads to drift phenomena.
The known measuring cell responds to all gases influencing the pH value of the electrolyte so that its selectivity for measurements in corresponding gas mixtures is not adequate. Each potentiometric pH measurement by means of a glass electrode is based on a characteristic which is nonlinear, that is logarithmic. This characteristic must be linearized in a complex manner for obtaining better evaluation possibilities.